The invention relates in general to thrust engines and in particular to a new and useful arrangement to cool the thrust nozzle for a rocket engine whose rear portion is cooled by at least one liquid medium, in particular a liquid fuel or propellant such as hydrogen which evaporates within the wall of the rear thrust nozzle section and discharges into the open under thrust generation (rear open cooling circuit).
According to German Pat. No. DE-PS 23 56 572, a rocket combustion chamber with thrust nozzle is known, which has two mutually independent cooling circuits, a regenerative front one which is coordinated with the front section of the thrust nozzle as well as to the combustion chamber and leads to the injection head, and a rear one which flows in the rear thrust nozzle section and is open at the end of the thrust nozzle, its flow volume being very small compared to the regenerative cooling circuit and the evaporating volume inside the rear thrust nozzle wall, discharging into the open under thrust generation.
It is further known in rocket combustion chambers with thrust nozzles to protect the nozzle neck and the rear section of the thrust nozzle by film cooling in contact with the inside of the nozzle wall, in addition to regenerative wall cooling, (see U.S. Pat. No. 3,605,412).
It is apparent from a comparison of the two above described cooling possibilities that both pure dump cooling and film or mist cooling have their advantages and disadvantages. For instance, dump cooling brings with it heavy nozzle designs because the nozzle must be dual-walled with mutual supports of inner and outer walls, or must consist of individual, juxtaposed tubes. Furthermore, the rear thrust nozzle components intended for dump cooling are very expensive due to their complicated design because, as already mentioned, they are often composed of individual tubes which are costly per se, with much manual labor required to join them firmly. Even though the cooling efficiency of dump cooling is high due to evaporation which consumes large amounts of heat, an "impulse" loss is associated with this cooling mode because the dump quantity does not participate in the reaction process with its high specific impulse generation.
The advantage of film cooling lies in light-weight nozzle design, but this cooling mode requires a great deal of medium or fuel because the mixing rate between the film mist and the radially adjacent, extremely hot thrust nozzle flow is very high so that coolant is lost continuously. Yet, there is not always assurance that the comingling coolant will participate in the reaction because the radial outside zone of the thrust nozzle flow is enriched by the coolant component. This reaction loss, in turn, leads to an overall process power loss. If an inert coolant is used, it is entirely at the expense of the specific impulse.
Thus, besides certain advantages, the above described cooling modes also have disadvantages which must not be overlooked and which, in dump cooling, have an additional effect and entail a functional limitation because it is no longer possible to increase the flow velocity inside the cooling channels due to the occurring high pressure losses and the attainment of the speed of sound.